104/ COLLISION OF STEEL WITH HARD BODIES. 



collifion in the atmofphere very vivid fparks ; which were 

 chiefly white, from the combuflion of the particles of the 

 Heel; but fometimes mixed with a few red fparks from, the 

 combuftion of the particles of the pyrites. The gunlock was 

 introduced under the receiver of an air pump, and the exhauf- 

 tion was made till the mercury in the fliort gage flood at about 

 -i% of an inch. The lock was then mapped, but no light 

 whatever was perceived ; and the phenomenon was uniform, 

 every precaution being taken to render the room dark, and to 

 preferve the apparatus in order. 

 Queftlon. Can IV. It is well known that in common cafes the fineft fteel 



tfnot firft whkc w * re ( * oes not ^ urn Wli ^ a wrute % nt or fparks in the atmof- 

 hot ? phere, unlefs it have been previoufly heated to a degree much 



above that of the red heat ; it confequently at firft view ap~ 

 pears extraordinary, that the particles feparated from the gun- 

 lock mould be heated fo as to burn vividly in air, and yet not fo 

 as to appear ignited in vacuo ; for it is not eafy to conceive 

 that they emit light, which from the minutenefs of their vo- 

 If it cannot, lume * cannot be perceived ; or to fuppofe that the opacity of 

 whfteheatf 1S the metallic fubflances fhould hinder light generated at their 

 in vacuo? points of contact from being vifible. I had formerly fuppofed, 



in reafoning upon the phenomenon of the collifion of flint and 

 flee], that f heat and light might in common inflances'be only 

 accidentally coexiflent ; and that in certain cafes very high 

 temperatures might be produced without caufing the appear- 

 ance of light. At prefent however I am inclined to believe, 

 that the phenomena may be adequately accounted for upon 

 principles that coincide with the common facts relating to the 

 production and communication of heat. 

 Explanation. Mr. Stodart % has fliewn, that when fteel is gradually 



Oxidation at heated it begins to change colour at about 430 9 Fahrenheit, 

 below ignition, And tn * s change of colour is occafioned by its combination 

 may devclope with oxigen, and, as there is every reafon to believe, mull 

 ca^hc conducl- ^ e conne & e d w,tn * ne evolution of heat. At about 600°, a 

 ed off, and may temperature much below that of ignition, it oxidates rapidly, 

 £■1 fliiMi and becomes covered with a bluifhgrey coating. And though 

 in thefe cafes of oxidation the heat evolved at the furface of 



* Or the fhort time of its emiffion.—- N. 



f Nicholfon's Journal, 4to, Vol. III. p. 517. 



J Nicholfon's Journal, 4to, Vol. IV. p. 130. 



the 



